1. Field of the Invention
This invention relates to electrical apparatus and more particularly to devices normally employed as part of a gaseous discharge lighting system, wherein this invention generates voltage pulses which are required and suppresses voltage surges which are potentially destructive.
2. Description of the Prior Art
A gaseous discharge lamp operates in conjunction with a ballast circuit of one or another form for the purposes of maintaining proper operation of the lamp. It is characteristic of these lamps that a voltage significantly in excess of the normal operating voltage of the lamp is required in order to initiate electrical conduction through the gaseous volume of the lamp. It is also characteristic of these lamps that once conduction has been initiated, a means must be provided to limit the current through the lamp, without which current-limiting means, commonly referred to as a ballast, the lamp itself and associated equipment would be destroyed.
Gaseous discharge lamps are normally intended to be operated from common alternating current power sources. The ballasting apparatus typically consists of various combinations of inductors, capacitors, and transformers, all of which are configured so as to maintain conduction through the lamp while limiting the current to that required for proper operation of the lamp. Since gaseous discharge lamps require a starting voltage in excess of their normal operating voltage, an auxiliary circuit, commonly referred to as an ignitor or starter is incorporated into the ballasting apparatus, frequently taking the form of one or more taps on an inductor or transformer and some means of electrically pulsing this circuit such that high voltage pulses are applied to the lamp terminals for the purpose of meeting the lamp's starting voltage requirements. Once conduction through the lamp has been initiated by the high voltage pulses, the normal operating currents and voltages required for proper operation of the lamp are maintained by the other ballasting components. These other ballasting components typically consist of various combinations of inductors, transformers, resistors, and capacitors. For various practical reasons, the ballast-lamp circuit is usually an inductive circuit; that is, the current through the lamp is primarily controlled by the inductance of the circuit. Any capacitors incorporated into the ballast serve to modify the voltage-current phase relationship presented to the power source and/or to modify the voltage-current waveforms presented to the lamp.
It is characteristic of the ignition pulses generated by ignitor circuits that the pulses exhibit short rise time to high voltages. As a consequence of utilizing a portion of the associated ballasting equipment as a transformer for generating these quickly rising high voltages, corona discharges are initiated within, between, and on the surfaces of electrical insulating materials used in the construction of the associated ballasting equipment.
This corona discharge has two serious effects which significantly reduce the operational life of the overall gaseous discharge lighting system, as well as aggravates and intensifies both conduction and radiated RFI emissions, and significantly increases the cost of fabrication and utilization of the gaseous discharge lighting equipment.
The first effect is simply the power demand imposed by the corona discharge itself. It has been found that in a typical gaseous discharge lighting apparatus, two-thirds of the energy available from the ignitor circuit is consumed by the corona discharge, the remaining energy from the ignitor circuit being available to satisfy the energy losses of the ignitor circuit itself and the starting requirements of the gaseous discharge lamp.
These losses require the ignitor circuit to be much larger and more powerful than would be required to merely start conduction in the lamp, as well as significantly increasing the stresses upon the components of the ignitor circuit.
The second effect of the corona discharge is that it destroys the insulation of the ballasting apparatus and its associated wiring. This severely limits the useful lifetime of the gaseous discharge lighting system, as well as significantly increases the material costs associated with the system in the attempt to provide increased life by merely using materials of high voltage ratings.
The presence of inductive components in the discharge lamp circuit, through which the normal operating currents of the lamp flow, allow the generation of extraordinarily large voltage surges as a consequence of failure of the lamp or electrical connections in the lamp circuit. It is characteristic that a short circuit is of no consequence. The normal operation of a gaseous discharge lamp requires the ballasting apparatus to be designed to operate into a short. They are all designed to do this. The same ballasting apparatus, however, utilizes inductive components to control the flow of current through the lamp. Various component failures in the gaseous discharge lamp, its socket, associated wiring, etc. will cause the rapid interruption of this current flow.
This rapid interruption of current flow in the lamp and ballast circuit causes very large voltage surges of the form E=L di/dt, where E is the voltage units of volts, L is the circuit inductance in units of henries, and di/dt is the rate of change of the current, in units of amperes for i and units of seconds for t. As an example, if a current of one ampere is flowing in a circuit which contains one henry of inductance and this current is interrupted in a time of one microsecond, a voltage of one million volts will be generated, this voltage rising at the rate of one trillion volts per second and obviously in excess of what can be withstood safely and/or without damage by any practicable system of insulation.
A further consequence of these circuit interrupting type failures is they frequently are of a rectifying nature; that is, they have the nature of an electrical diode. This phenomenon is called a rectifying arc. This causes large direct current ("DC") voltages to build up within the capacitors frequently employed in the ballasting apparatus, which are designed and rated for alternating current ("AC") operation and in any case incapable of withstanding the large DC voltages which typically appear, particularly since these abnormal DC voltages coexist with and add to the normal AC voltages impressed upon the capacitor or capacitors of the ballasting apparatus resulting in failure of the capacitors from overvoltage.
Another consequence of the rectifying arcs is that they cause the magnetic materials used in the construction of the inductive portions of the ballasting apparatus to saturate. When this happens, the inductance is no longer capable of controlling or limiting the current through the lamp or itself and the current thus increases to large values only limited by incidental resistances in the circuit. This phenomenon is also self-accelerating in that a slightly rectifying arc will tend to saturate the inductance in such a manner that the resultant abnormal current flow not only intensifies the arc, it becomes even more of a rectifier or diode, leading to a more intense and more rectifying arc, and so on until the ballast inductor and lamp are both destroyed.
Many types of gaseous discharge lamps incorporate in their construction an outer envelope inside of which is placed the inner envelope within which the gaseous conduction itself takes place. For principally thermal reasons, the interstitial space between the inner and outer envelopes is typically evacuated to a high degree, that is, contains a vacuum. Electrical connections between the inner envelope's electrodes and the electrical lead wires which penetrate the outer envelope are typically made by a spot-welding or crimping operation. Due to the high operating temperatures and temperature cycling normally associated with the inner envelope and its electrodes, these spot-welded or crimped connections frequently become loose, thus opening the circuit and cutting off the flow of current in the circuit formed by the gaseous discharge lamp and the inductive ballasting components. Since these connections exist within the vacuum space of the outer envelope, this interruption of current flow occurs almost instantaneously, in accordance with the operating characteristics of the well-known high vacuum hard-contact switch. This rapid current-flow interruption, due to the typical inductance of the ballast, generates extremely high voltages in accordance with the E=L di/dt relationship. These interruptions also tend to occur repeatedly and rapidly due to the small gap typically present in between the surfaces of the failed connection within the vacuum envelope.
Ballasting circuits which utilize a portion of an inductive component as a transformer also invariably couple significant amounts of radio frequency type energy back into AC power lines used as a source of power for the lighting equipment. This coupled energy and the corona discharge are the major sources of the notorious conducted and radiated radio frequency interference ("RFI") associated with gaseous discharge lighting equipment. Traditional ignitor and ballasting circuit configurations preclude the use of any practicable insulation, transient suppression, or RFI suppression.
As a result of all the above, gaseous discharge lighting installations today suffer from high failure rates, unnecessarily expensive construction, and generate unacceptable levels of radio frequency interference.
Therefore, the features of the prsent invention efficiently generate high voltage starter pulses, without inducing any corona discharge in itself or in the associated ballasting equipment or its wiring, effectively prevent the occurrence of destructive voltage surges due to derangement of the associated gaseous discharge lamp and/or its electrical current-carrying hardware, and permit the suppression of any radio frequency interference by the simple application of well-known inexpensive shielded wiring techniques.